1. Field of the Invention
The present invention relates to a two-lead type heat-sensor complex comprising a heater unit and a sensor unit as integrated. More particularly, the invention relates to a heater-sensor complex suitable for use as a heat input for a soldering iron.
2. Description of the Related Art
Referring to FIG. 11 which illustrates the tip of a soldering iron, a rod-shaped ceramic heater 51 is shown as fitted in the tip portion 52 of a soldering iron. This ceramic heater 51 includes a ceramic green sheet 53 wrapped round a ceramic mandrel. The ceramic green sheet 53 has been printed with a heater pattern and a sensor pattern. The topological relationship of the patterns may for example be as illustrated in FIG. 12. In this example, the sensor pattern 55 is a pattern printed using a tungsten paste in a breadth of 0.2 mm and a heater pattern 54 is a pattern printed using the tungsten paste in a breadth of 0.4 mm surrounding said sensor pattern 55. The sensor pattern 55 extends rearward across the heater pattern 54 and is connected to sensor electrodes 56, while the heater pattern 54 is connected to heater electrodes 57 which are situated forwardly of said sensor electrode 56.
However, the sensor-mounted ceramic heater illustrated in FIGS. 11 and 12 has the disadvantage that because it has four leads, the outer diameter of said ceramic mandrel cannot be reduced to less than 3.5 mm. Thus, if a size reduction is required of a soldering iron, the pipe diameter cannot be decreased any further because of the limited freedom in management of the leads. There also is the disadvantage that because the sensor is located on the peripheral surface of the ceramic mandrel, the tip temperature of the soldering iron cannot be detected with precision. Furthermore, in the case of soldering irons such that the detection of the tip temperature hinges on the temperature-dependent change in resistance value, the illustrated soldering iron being no exception, there must be a fair variation among products so that each product has to be adjusted to fall within tolerance limits.
Meanwhile, there is also available a heater-sensor fabricated in such a manner that instead of wrapping a green sheet round a ceramic mandrel, an electrothermic wire is directly wound round a ceramic mandrel and the winding is then coated with a ceramic coating to insure the necessary insulation.
However, such a heater has the disadvantage that when the ceramic coat consists of closely packed fine ceramic particles, the coat tends to develop cracks because of the relatively high coefficient of thermal expansion of the electrothermic wire as compared with the coefficient of thermal expansion of the ceramic coat. On the other hand, if coarse ceramic particles are used for said ceramic coating, the insulation resistance drops to cause a leak of current to the work load, e.g. a substrate.
Designed to overcome the above-mentioned disadvantages of the prior art, the present invention has for its object to provide a heater-sensor complex which, despite its simple construction, is not only capable of detecting the tip temperature of a soldering iron with precision but also insures a thorough insulation without the risk for incidence of cracks in the insulation layer.